[Problem] To provide a new pharmacological action for high-molecular-weight polyphenols [Solution] To provide high-molecular-weight polyphenols extracted from fermented tea (e.g. oolong tea or black tea), which are high-molecular-weight polyphenols with a number average molecular weight ranging from 9,000 to 18,000, and containing in their partial structures a procyanidin structure of polymerized catechins and/or their gallates, and a structure of linkage between the B-rings of catechins and/or their gallates. These high-molecular-weight polyphenols have the functions of mitochondrial activation, suppression of increase in blood glucose, and suppression of weight gain. Therefore, they can be used as drugs for the treatment of mitochondrial disease, and also as drugs for the prevention and treatment of diabetes. They may also be added to health foods and beverages. The selected drawing shows changes in blood glucose level when the said high-molecular-weight polyphenols are given daily to diabetes model mice in specified doses.